1. Field of the Disclosure
The invention relates to a method for producing glasses, in particular LAS glasses and alkali-free aluminosilicate glasses, as well as glasses for the production of glass ceramics. The invention also relates to glasses and glass ceramics, and the use of same.
2. Description of Related Art
For the production of glass, a mixture or batch is introduced into a furnace melting tank and the batch is melted, the mixture first being converted to the stage of the batch agglomeration phase, which is also designated the raw melting, which describes the melting process of the batch.
A batch cover forms thereby, underneath which the melt moves in the form of a counterclockwise principal flow vortex. A hot melt flow partially detaches from this flow vortex and rises toward the top. This point is called the thermal source point. The source point in a furnace melting tank marks the transfer from the first region into the second region of the furnace tank.
In “Glastechnische Fabrikationsfehler [Technical Glass Manufacturing Defects]”, edited by H. Jepsen-Marwedel and R. Brückner, 4th Edition, Springer Publ. Co., it is described that under the influence of high temperature in the furnace melting tank, a thin melt layer, the thickness of which amounts to only several millimeters and which flows off under the effect of gravity, is formed on the surface of the batch. Due to gases erupting from the inside the batch, which form large bubbles and can cause the melt layer to appear full of holes, the newly formed glass melt is pushed away from the batch.
The batch is essentially heated and melted by the flow of glass penetrating below the batch carpet. The reaction gas formed at the hot melt front on the underside of the batch penetrates into the porous batch layer and flows to the top through the hollow spaces.
The increase in temperature inside the batch layer proceeds slowly, so that sufficient time remains for the course of the melt reactions. The reactions in the batch agglomeration phase are different for individual glass systems. In general, however, at first, due to solid-solid reactions, the more reactive components form solid solutions and eutectic phases, which then also accelerate other reactions between the less-reactive batch components due to the formation of the melt.
During the raw melting, up to approximately 1400° C., silicate-forming reactions are concluded and subsequently, the remaining quartz grains, Al2O3 grains, and zirconium-containing grains are dissolved. In addition to temperature, the quantity of undissolved grains and their size represent determining factors for the rate of dissolution.
Shards, preferably shards specific to the glass type, can be added to the batch in a concentration of up to 50% or more.
Up to 20 wt. % gases, which are bound to the raw materials, are introduced into the furnace melting tank with the batch. Due to the decomposition of these raw materials, particularly the carbonates, huge amounts of gases are released, the principal amount of which is discharged into the furnace atmosphere during the batch agglomeration phase and the raw melting. The remainder of approximately 0.001 to 0.1 vol. % of the evolved quantity of gas still remaining in the form of bubbles after the raw melting, as well as the gases remaining dissolved in the melt, must be removed or must be reduced to an extent that is no longer disruptive during the subsequent refining process.
Primarily during the process of dissolving sand grains and zirconium-containing grains, small gas bubbles arise on them that must also be removed from the glass melt.
The object of the refining is to remove bubbles that are still present, to reduce the concentration of dissolved gases, which could give rise to post-gases, and to homogenize the melt. For this purpose, thermal, mechanical, and chemical refining methods or a combination thereof are used in glass technology.
All technical-process measures for refining have the objective of decreasing the rise velocity v of bubbles and thus the time for the bubbles to rise. The rise velocity v of bubbles with a diameter d is given according to Stokes by:
  v  =            1      18        ⁢                            g          ⁢                                          ⁢          ρ          ⁢                                          ⁢                      d            2                          η            .      (g: acceleration due to gravity; ρ: density of the glass melt, η: viscosity of the glass melt)
In order to increase the bubble rise velocity, two parameters can be changed essentially: the diameter of the bubbles d can be increased (very effective due to d2) and/or the viscosity of the glass melt can be reduced by increasing the temperature in the refining region.
It is described in DE 199 39 771 A1 that in general two principal refining methods are known, which differ essentially by the type and manner of producing the refining gas.
In the physical refining method, the viscosity of the glass melt is reduced by increasing the temperature. Therefore, in order to reduce the viscosity during the refining, higher temperatures are established in the glass melt than in the melting and standing regions. The higher the refining temperature can be selected in each case, the more effective is the removal of bubbles from the melt. In this case, the viscosity of the melt should be <102 dPa·s as much as this is possible. The maximum permissible refining temperature, however, is limited by the temperature resistance of the wall material of the melting aggregate used each time, and is approximately 1720° C. in conventional furnace melting tanks.
Most frequently chemical refining methods are used. The principle here is that compounds are added to the batch that can either decompose and give rise to gases or which are volatile at higher temperatures, or which deliver gases in an equilibrium reaction at higher temperatures. These respective gases diffuse into the bubbles that are present and enlarge them. For example, sodium sulfate that is used, e.g., for the refining of soda-lime glasses belongs to the first group of compounds. In this case, SO2 and O2 are delivered in a temperature range of 1100° C. to 1450° C. with a maximum at 1380° C. This temperature range approximately corresponds to the refining range of such glasses.
By way of example, sodium chlorides belong to the second group of compounds, and polyvalent ions such as As2O3 or SnO2 belong to the last group of compounds.
DE 10 2009 021 116 A1 discloses a method for producing borosilicate glasses with the use of vanadium pentoxide (V2O5) as a refining agent. The glasses produced are only designated as bubble-poor.
Glasses for the production of transparent, colored glass ceramics, for the production of which SnO2 or sulfate compounds, among others, are used as refining agents, are known from DE 199 39 787 A1. These refining agents are utilized as replacements for the refining agents, arsenic oxide or antimony oxide. The high-temperature refining occurs at temperatures of more than 1975° C. Information on the number of bubbles obtained, however, is not given for glasses containing these types of refining agents.
It has been shown that with the use of sulfates during high-temperature refining above 1750° C., spontaneous new bubble formation occurs (so-called reboil bubbles) due to the greatly increasing partial pressures of O2 and particularly of SO2 to >5 bars. The low bubble concentration achieved in the upstream refining stages increases again thereby, so that a bubble concentration of >2/kg results in the product.